FIG. 1A shows an example of the configuration of a conventional optical wavelength-division multiplex(ing) (WDM) access system (Japanese Patent Application Laid-open No. 2000-196536). In FIG. 1A, a center apparatus (OLT) 50 and a wavelength multi/demultiplex(ing) apparatus 60 are connected together in a multiplex section via multiplex section optical fibers 1 and 2. The wavelength multi/demultiplex(ing) apparatus 60 and a plurality of optical network units (ONUs) 70-1 to 70-n are connected together in an access section via access section optical fibers 3 and 4. In this case, one wavelength band D is assigned to downlink signals from the OLT to the ONUs. One wavelength band U (≠ED) is assigned to uplink signals from the ONUs to the OLT. In the example shown below, wavelengths d1 to λd1 to λdn in the wavelength band D and wavelengths λu1 to λun in the wavelength band U are assigned to the respective ONUs.
A transmission section (S) 51 of the OLT 50 multiplexes the wavelengths of a downlink optical signal within the wavelength band D (λd1 to λdn) and of an optical carrier for an uplink signal within the wavelength band U (λu1 to λun). The transmission section (S) 51 then transmits the resulting signals to the wavelength multi/demultiplex(ing) apparatus 60. The wavelength multi/demultiplex(ing) apparatus 60 divides the downlink optical signal within the wavelength band D and the optical carrier for the uplink signal within the wavelength band U into the respective wavelengths. The wavelength multi/demultiplex(ing) apparatus 60 transmits, via the access section optical fiber 3, the pairs of the downlink optical signals of the wavelengths λd1 to λdn and uplink optical carrier of the wavelengths λu1 to λun obtained by the division and corresponding ONUs 70-1 to 70-n.
The ONU 70-1 uses a WDM coupler 71 to demultiplex the downlink optical signal of the wavelength λd1 from the optical carrier for the uplink signal of the wavelength λu1, the signal and carrier having been transmitted. The ONU 70-1 further uses an optical receiver (R) 72 to receive the downlink optical signal of the wavelength λd1. The ONU 70-1 also uses an optical modulator (M) 73 to obtain an uplink optical signal from the optical carrier for the uplink signal and then transmits the signal to the wavelength multi/demultiplex(ing) apparatus 60 via the access section optical fiber 4. This also applies to the other ONUs. The wavelength multi/demultiplex(ing) apparatus 60 multiplexes the wavelengths of the uplink optical signals of the wavelengths λu1 to λun transmitted by the ONUs. The uplink optical signals wavelength-multiplexed are transmitted to the OLT 50 via the multiplex section optical fiber 2. A reception section (R) 52 then receives the signals.
In this case, as shown in FIG. 1B, the wavelength band D (wavelength λd1 to λdn) for the downlink optical signals and the wavelength band U (wavelength λu1 to λun) for the uplink optical signals (the optical carrier for the uplink signal) are arranged so as not to overlap on a wavelength axis. An arrayed waveguide grating (AWG) 61 used as the wavelength multi/demultiplex(ing) apparatus 60 has a function for demultiplexing and sending a downlink signal wavelength (for example, λd1) and uplink signal wavelength (for example, λu1) within an FSR (Free Spectral Range) to the same port. A pair of a downlink optical signal having a wavelength within the wavelength band D and an uplink optical signal having a wavelength within the wavelength band U are input to each ONU. Thus, as shown in FIG. 1C, by using WDM couplers 71 conforming to the same specifications and which separate the wavelength bands D and N from each other, it is possible to separate the downlink optical signal from the uplink optical signal to prevent their mutual interferences.
A method has been proposed with which when the transmission section 51 of the OLT 50 transmits optical carriers for uplink signals of the wavelengths λu1 to λun, broadband light containing the wavelengths λu1 to λun is used and with which the AWG 61 of the wavelength multi/demultiplex(ing) apparatus 60 spectrum-slices the light to obtain optical carriers for uplink signals of the wavelengths λu1 to λun, which are then supplied to the ONUs (Japanese Patent Application Laid-open No. 2001-177505).
Such improvements are intended to allow the ONUs 70-1 to 70-n to be composed of common devices (a reduction in the number of types of devices). Specifically, the OLT 50 supplies optical carriers for uplink signals of the respective wavelengths to the corresponding ONUs. Thus, the ONUs need not comprise the respective light sources for the assigned wavelengths. The ONUs can utilize optical modulators 73 conforming to the same specifications, for the wavelength band U. Moreover, one downlink signal within the wavelength band D and one uplink signal within the wavelength band U are input to each ONU. The ONUs can use the WDM couplers 71 conforming to the same specifications and which separate the wavelength bands D and U from each other, in order to separate the downlink optical signal from the optical carrier for the uplink signal.
Further, in a configuration in which the OLT 50 is located opposite the plurality of ONUs 70-1 to 70-n via the AWG 61 or multiport wavelength filters as shown in FIG. 2A, the use of a wavelength variable light source as an optical transmitter (S) 75 makes it possible to arrange optical transmitters conforming to the same specifications, in the ONUs. In this case, the ONUs 70-1 to 70-n transmit uplink optical signals of the different wavelengths λu1 to λun; FIG. 2B shows the wavelength characteristics of the ONUs.
Moreover, similarly, in connection with a configuration shown in FIG. 2A, a proposal described below has been proposed (Akimoto, K, et al., “Spectrum-sliced, 25-GHz spaced, 155 Mbps×32 channel WDM access”, The 4th Pacific Rim Conference on Lasers and Electro-Optics, 2001 (CLEO/Pacific Rim 2001), Vol. 2, pp. II-556-557). The optical transmitter 75 of each ONU modulates broadband light having a wide spectrum width in the wavelength band U; FIG. 2C shows the wavelength characteristics of the optical transmitter 75. Each ONU modulates the broadband light to obtain an uplink optical signal and then transmits the signal. The wavelength multi/demultiplex(ing) apparatus 60 spectrum-slices the uplink optical signal to multiplex their wavelengths and then transmits the resultant signals to the OLT 50. This configuration is substantially equivalent to that in which the ONUs transmit uplink optical signals of different wavelengths. However, this configuration is characterized in that optical transmitters conforming to the same specifications can be arranged in the ONUs.
It is possible to use an electric signal to directly modulate a superluminescent diode or a semiconductor optical amplifier (SOA) in order to obtain modulated light of a large optical spectral width. Alternatively, it is possible to use an external modulator to modulate output light (broadband non-modulated light) from a semiconductor optical amplifier or an erbium-doped fiber amplifier (EDFA).
If ONUs accommodated in the OLT are added to a conventional optical wavelength-division multiplex(ing) system, different wavelength bands are set for the wavelengths assigned to the standard ONUs and for the wavelengths assigned to the additional ONUs. In general, a configuration such as the one shown in FIG. 3 is possible. Basically, in this configuration, a standard and additional versions of each component are arranged in parallel; the components include the transmission section 51 and reception section 52 of the OLT 50, the AWG 61 of the wavelength multi/demultiplex(ing) apparatus 60, and the ONUs 70-1 to 70-n, all of which are shown in FIG. 1.
In this case, the wavelengths λd1 to λdn of a wavelength band Da for downlink signals and the wavelengths λu1 to λun of a wavelength band Ua for uplink signals are assigned to the standard ONUs 70-1 to 70-n. Further, the wavelengths λdn+1 to λdn+m of a wavelength band Db for downlink signals and wavelengths λun+1 to λun+m of a wavelength band Ub for uplink signals are assigned to additional ONUs 70-n+1 to 70-n.
The OLT 50 comprises a standard transmission section (Sa) 51a that multiplexes the wavelengths of and transmits downlink optical signals in the wavelength band Da (λd1 to λdn) and optical carriers for uplink signals in the wavelength band Ua (λu1 to λun). The OLT 50 also comprises an additional transmission section (Sb) 51b that multiplexes the wavelengths of and transmits downlink optical signals in the wavelength band Db (λdn+1 to λdn+m) and optical carriers for uplink signals in the wavelength band Ub (λun+1 to λun+m). Moreover, the OLT 50 comprises a standard reception section (Ra) 52a that receives optical signals in the wavelength band Ua (λu1 to λun) and an additional reception section (Rb) 52b that receives optical signals in the wavelength band Ub (λun+1 to λun+m).
A WDM coupler 53d multiplexes the wavelengths of standard and additional downlink optical signals and optical carriers for uplink signals transmitted by the standard transmission section 51a and the additional transmission section 51b. The WDM coupler 53d then transmits the resultant signals to the wavelength multi/demultiplex(ing) apparatus 60 via the multiple section optical fiber 1. The wavelength multi/demultiplex(ing) apparatus 60 uses a WDM coupler 62d to separate the standard wavelength bands Da and Ua from the additional wavelength bands Db and Ub. The wavelength multi/demultiplex(ing) apparatus 60 uses AWGs 61a and 61b to divide the wavelength bands separated, into downlink optical signals and optical carriers for uplink signals of the respective wavelengths. Pairs of downlink optical signals of the wavelengths λd1 to λdn and optical carriers for uplink signals of the wavelengths λu1 to λun which have been divided by the AWG 61a are transmitted to the corresponding ONUs 70-1 to 70-n via the access section optical fiber 3. Pairs of downlink optical signals of the wavelengths λdn+1 to λdn+m and optical carriers for uplink signals of the wavelengths λun+1 to λun+m which have been divided by the AWG 61b are transmitted to the corresponding ONUs 70-n+1 to 70-n+m via the access section optical fiber 3.
WDM couplers 71a of the standard ONUs 70-1 to 70-n are equally characterized by demultiplexing the wavelength bands Da and Ua. Optical modulators 73a of the standard ONUs 70-1 to 70-n are equally characterized by modulating optical carriers of the wavelength band Ua. On the other hand, WDM couplers 71b of the standard ONUs 70-n+1 to 70-n+m are equally characterized by demultiplexing the wavelength bands Db and Ub. Optical modulators 73b of the standard ONUs 70-n+1 to 70-n+m are equally characterized by modulating optical carriers of the wavelength band Ub. Uplink optical signals transmitted by each pair of ONUs are transmitted to the AWGs 61a and 61b of the wavelength multi/demultiplex(ing) apparatus 60 via the access section optical fiber 4. Then, a WDM coupler 62u demultiplexes the standard and additional uplink optical signals wavelength-multiplexed. The WDM coupler 62u then transmits the resultant signals to the OLT 50 via the uplink multiple section optical fiber 2. The OLT 50 uses a WDM coupler 53u to separate the standard wavelength band Ua from the additional wavelength band Ub. The standard and additional reception sections 52a and 52b receive the wavelength bands separated.
A wavelength-division multiplex(ing) access system shown in FIG. 3 is obtained simply by expanding the conventional wavelength-division multiplex(ing) access system shown in FIG. 1A. As shown in FIG. 4A, the wavelengths λd1 to λdn of the wavelength band Da are assigned to standard downlink signals. The wavelengths λun+1 to λun+m of the wavelength band Ua are assigned to standard uplink signals. The wavelengths λdn+1 to λdn+m of the wavelength band Db are assigned to additional downlink signals. The wavelengths λun+1 to λun+m of the wavelength band Ub are assigned to standard downlink signals. Further, the wavelength bands Ua, Da, Ub, and Db are assigned to the wavelength axis on this order.
With this assignment, as shown in FIG. 4B, the transmission characteristics of the WDM couplers 53d and 53u of the OLT 50 and of the WDM couplers 62d and 62u of the wavelength multi/demultiplex(ing) apparatus 60 may be such that they can merge or separate the standard wavelength bands Ua and Da with or from the additional wavelength bands Ub and Db. Accordingly, all of these components may conform to the same specifications.
However, the WDM couplers 71a of the standard ONUs 70-1 to 70-n separate the wavelength bands Ua and Da from each other. The WDM couplers 71b of the additional ONUs 70-n+1 to 70-n+m separate the wavelength bands Ub and Db form each other. Accordingly, these components require different transmission characteristics as shown in FIGS. 4C and 4D. Similarly, the operating bands of the standard and additional optical modulators 73a and 73b are different, the standard and additional optical modulators 73a and 73b operate in the wavelength bands Ua and Ub, respectively, as shown in FIGS. 4E and 4F. In other words, for the standard and additional ONUs, the WDM couplers 71a and 71b must conform to different specifications, and the optical modulators 73a and 73b conform to different specifications.
Common devices can be used as the WDM couplers 71a and 71b if they have a transmission characteristic such that the wavelength bands Da and Ub can be separated from the wavelength bands Ua and Db, shown in FIG. 4G. However, different optical receivers and different connection ports must be used for the standard and additional WDM couplers and the standard and additional optical modulators, respectively. Thus, this configuration does not actually allow all the ONUs to be composed of common devices. Further, common devices can be used as both optical modulators 73a and 73b provided that they operate over a broad band spanning the wavelength bands Ua, Da, and Ub. However, the operating band is now limited to about several 10 s of nm. Accordingly, the number of wavelengths (ONUs) is not enough to allow the optical modulators 73a and 73b to be composed of only one type of component. These light emission bands are also a limiting factor for a configuration in which an optical transceiver 75 that modulates a wavelength variable light source or a broadband light is placed in each ONU as shown in FIG. 2B or 2C.
It is an object of the present invention to provide a wavelength-division multiplex(ing) access system which allows standard and additional ONUs to conform to the same specifications and which can minimize an operating band for an optical modulator or a light emission band for an optical transmitter.